Environmental stimuli and physiological responses: The current view on electrical signalling

Abstract Electrical signals have been studied in numerous species so far. It appears that two main types of such signals occur in plants, rapid action potentials (APs) and slower variation potentials (VPs). While APs are generally evoked by non-invasive stimuli and follow the all-or-nothing principle as in neurons, VPs are mostly triggered by wounding and do not follow the all-or-nothing law. They are correlated to the stimulus strength and last longer than APs. The transmission of both, APs and VPs, occurs via the phloem over long distances and via plasmodesmata over short distances from cell to cell. Regarding physiological functions of electrical signals, numerous examples exist. They regulate rapid leaf movements in order to catch insects and for instance, affect nutrient uptake, gene expression and phloem transport. Recently, it was shown that apart from hydraulic signals, electric signals also play a significant role in root-to-shoot communication of drought-stressed plants. Re-irrigation of plants after soil drying initiates rapid hydraulic as well as electric signalling which affects the gas exchange of leaves. In addition, evidence was found for a link between electrical signals and photosynthesis as well as respiration. Wound-induced VPs cause a transient suppression in photosynthetic activity and an increase in respiratory CO2 release. The results led us to conclude that different stimulation types trigger characteristic electrical signals each with specific influence on physiological processes.